Smelt cyclones are typically used in the pre-reduction and melting of metalliferous material, such as iron oxides contained in iron ore. A smelt cyclone is a vessel defining a cylindrical chamber provided with means to inject metalliferous material and means to inject oxygen or an oxygen containing gas from a number of locations around the circumference of the chamber. Such smelting cyclones are known for instance from EP0726326 and EP0735146.
The metalliferous material and oxygen are injected in an about tangential direction therewith generating a vortex or whirling flow inside the cyclone. At the same time a reducing process gas is introduced into the cyclone which in combination with the injected oxygen is partly combusted resulting in sufficiently high temperatures to melt the metalliferous material. The vortex or whirling flow in the cyclone promotes mixing of the injected oxygen and the reducing process gas and also heat exchange with the metalliferous material. As a result of the swirl motion, particles of metalliferous material and molten metalliferous material are separated from the gas and collect on the wall of the cyclone wall from where they flow downward to accumulate in a vessel where final reduction takes place.
A known process for smelting a metalliferous material is referred to hereinafter as the “HIsarna” process. The process is carried out in a smelting apparatus that includes (a) a smelting vessel that includes solids injection lances and oxygen-containing gas injection lances and is adapted to contain a bath of molten metal and (b) a smelt cyclone for pre-treating a metalliferous feed material that is positioned above and communicates with the smelting vessel. The HIsarna process and apparatus are described in WO 00/022176.
The term “smelt cyclone” is understood herein to mean a vessel that typically defines a cylindrical chamber and is constructed so that feed materials supplied to the chamber move in a path around a vertical central axis of the chamber and can withstand high operating temperatures sufficient to at least partially smelt metalliferous feed materials.
In one form of the HIsarna process, carbonaceous feed material (typically coal) and flux (typically limestone, burnt lime or the like) are injected into a molten bath in the smelting vessel. Metalliferous feed material, such as iron ore is injected into and heated and partially melted and partially reduced in the smelt cyclone. This molten, partly reduced metalliferous material flows downwardly from the smelt cyclone into the molten bath in the smelting vessel and is smelted to molten metal in the bath. Hot, reaction gases (typically CO, CO2, H2, and H2O) produced in the molten bath are partially combusted by oxygen-containing gas (typically technical-grade oxygen) in an upper part of the smelting vessel. Heat generated by the post-combustion is transferred to molten material in the upper section that falls back into the molten bath to maintain the temperature of the bath. The hot, partially-combusted reaction gases flow upwardly from the smelting vessel and enter the bottom of the smelt cyclone. Oxygen-containing gas (typically technical-grade oxygen) is injected into the smelt cyclone via tuyeres that are arranged in such a way as to generate a cyclonic swirl pattern about a central axis of the chamber of the smelt cyclone. This injection of oxygen-containing gas leads to further combustion of smelting vessel gases, resulting in very hot (cyclonic) flames. Finely divided incoming metalliferous feed and flux materials are injected pneumatically into these flames via tuyeres in the smelt cyclone, resulting in rapid heating and partial melting accompanied by partial reduction (roughly 10-20% reduction). The metalliferous feed and flux are injected in such a way as to create additional cyclonic swirl, adding to the swirl created by the oxygen-containing gas. The reduction is due to CO and H2 in the reaction gases from the smelting vessel and also the heating of the hematite above is dissociation temperature. The hot, partially melted metalliferous feed material is thrown outwards onto the walls of the smelt cyclone by cyclonic swirl action and, as described above, flows downwardly into the smelting vessel below for smelting in that vessel.
The net effect of the above-described form of the HIsarna process is a two-step countercurrent process. Metalliferous feed material is heated and partially reduced by outgoing reaction gases from the smelting vessel (with oxygen-containing gas addition) and flows downwardly into the smelting vessel and is smelted to molten iron in the smelting vessel. In a general sense, this countercurrent arrangement increases productivity and energy efficiency.
Through development work, including a series of trials in a pilot plant, it was found that the heat flux/heat flux density on some water-cooled wall sections of the smelt cyclone is significantly less than other wall sections of the smelt cyclone. As a result of this a higher than normal amount of injected metalliferous feed material is entrained in off-gas leaving the smelt cyclone. This affects overall productivity of the HIsarna process because this feed material does not report to the smelting vessel and, therefore, does not contribute to molten metal production.